The Tennessee Valley Authority (TVA) undertook a program of research, development, and production of phosphate fertilizers soon after the agency was created by an Act of Congress in May, 1933. TVA's fertilizer program entailed phosphorus furnace smelting of phosphate ore that was available from deposits in middle Tennessee. The mineral is fluorapatite and in igneous and metamorphic deposits it is represented by the formula Ca10(PO4)6F2. The middle Tennessee deposits, however, are sedimentary, and they differ markedly from fluorapatite. The mineral is micro-crystalline and there is extensive substitutions of carbonate and fluorine for phosphate. The mineral in middle Tennessee deposits is called carbonate apatites or francolite.
In a TVA publication (Agglomeration of Phosphate Fines for Furnace Use, E. L. Stout, Chemical Engineering Report No. 4, 1950) it was reported that while two large-scale phosphorus furnaces were under construction, arrangement were made to secure natural lump phosphate from middle Tennessee deposits for smelting. Feed material for phosphorus furnaces must be in lump form. In the course of handling and drying this lump material, it was known that phosphate fines would be produced. The small sized phosphate could be ground to powder and acidulated with phosphoric acid to produce concentrated superphosphate fertilizer. The mining operation and prospecting carried on in middle Tennessee indicated that the supplies of lump rock from this source were being depleted rapidly and soon would be unavailable. Furthermore, several thousand tons of excess fines accumulated at the TVA plant in 1935.
TVA had to abandon plans to smelt naturally occurring lump phosphate as stated in Chemical Engineering Report No. 4.                “Since it was evident that the supply of natural lump rock that could be obtained in Tennessee was insufficient for continued large-scale furnace operation, it became necessary to consider methods for the agglomeration of the phosphate matrix [raw phosphate ore] or phosphate sands [beneficiated phosphate ore]. Several rotary lime kilns were on hand at Nitrate Plant No. 2 which had been built during World War I and had been turned over to TVA in 1933. Consideration was given to agglomeration of phosphate fines in these kilns by heating the fines to incipient fusion so that they would stick together because of the rotation of the kiln and would form nodules. After the process was studied on a small scale, agglomeration was carried out successfully in the lime kilns. The process called “nodulizing” has been used in the TVA plant since 1936 as a method for agglomerating phosphate fines for use in furnace charges.”        
Land in Maury County, Williamson County and Giles County in middle Tennessee had deposits of the mineral fluorapatite and TVA procured mineral rights in the three counties. Equipment was installed in Maury County to beneficiate the ore by hydrometallurgical separations; most of the clay in the ore was removed by washing and phosphate grains, called phosphate sand, were recovered. The beneficiated phosphate ore was heated in rotary kilns to the temperature range of 2600° to 2700° F. Clay remaining in the beneficiated ore after hydrometallurgical separations was melted and provided a liquid phase that served as a binder to agglomerate the small particles. Raw phosphate ore contained more clay and it was agglomerated by heating in the temperature range of 2200° to 2350° F.
In 1935 and 1936 prior to the operations of facilities to beneficiate raw phosphate ore, the feedstock at TVA was naturally occurring lump phosphate ore containing 31 to 33 percent P2O5. In 1937, 24.5 percent was lump phosphate and in 1938 only 6.1 percent was lump phosphate. Thus the change from lump phosphate to nodulized phosphate sand was completed in 1938.
A typical composition of feedstock prepared by agglomerating phosphate sand is given in Table 1.
TABLE 1Typical Composition of Nodulized Phosphate Sand@ConstituentPercentMoisture0.0P2O527.5CaO39.0SiO224.1Fe2O23.3Al2O34.2F2.2K2O0.8MgO0.3MnO20.3Na2O0.6S0.0@Raw phosphate ore was beneficiated by hydrometallurgical separation to remove most of the clay to obtain phosphate sand; beneficiated raw phosphate ore was heated in a rotary kiln to agglomerate the ore for phosphate furnace feedstock. 
As shown in Table 1 the nodulized phosphate sand contained 27.5 percent P2O5, as compared with 31-33 percent P2O5 for natural lump feedstock. The smelting characteristics of natural lump phosphate and nodulized phosphate sand was reported in a TVA publication, Production of Elemental Phosphorus by the Electric-Furnace Method, R. B. Burt and J. C. Barber, Chemical Engineering Report No. 3, Tennessee Valley Authority, 1952.                “When the phosphate was nodulized or sintered [agglomerated by heating on a traveling grate], the moisture, sulfur, carbon dioxide, organic matter, and 35 to 40 percent of the fluorine was volatilized. Although the nodules and sinter were porous and their structures appeared to be somewhat weak, furnace operation with these materials was markedly superior to the operation with uncalcined phosphate feeds [natural lump phosphate]. Furnace draft control was improved significantly when the nodules or sinter was fed; there was more uniform movement of charge in the furnace as a result of the absence of crusting and the minimum formation of fines. During tests at the original No. 2 furnace the furnace-gas temperatures were approximately 300° F. lower when nodules or sinter was used than they had been when uncalcined Tennessee phosphate (plate brown rock or broken rock) was fed. The electrical energy requirement was 10 to 15 percent less for the nodulized feed than it was for uncalcined Tennessee phosphate; this was a result of the improved furnace operation and the higher grade of the nodulized material. A decrease in electrical energy requirement of this magnitude was not realized when sinter was fed because the sinter was prepared from the mixture of washed phosphate sand and matrix [raw phosphate ore], and the P2O5 content of this material was lower than that of the nodules or uncalcined rock phosphate normally fed to the furnace.”The “grade” referred to above was the percent P2O5 in the phosphate-plus-silica rock. Beneficiation of raw phosphate ore by washing did not separate silica and the nodulized material contained 24.1 percent SiO2, as shown in Table 1. The SiO2 content of lump phosphates varied from 7.8 to 11.4 percent and more silica rock was added to the furnace burden for a flux than for raw phosphate ore that was beneficiated by washing. Thus the percent P2O5 in the phosphate-plus-silica rock was higher for nodulized feed than it was for the “uncalcined Tennessee phosphate” which was lump phosphate.        
TVA washed raw phosphate and separated clay from the ore by hydrometallurgy. The beneficiated ore was agglomerated by heating in rotary kilns to obtain feedstock for smelting in phosphorus furnaces. However, raw phosphate ore that was amenable to beneficiation in Maury County and Williamson County was exhausted by 1950; excessive quantities of mineral fluorapatite were lost when clay was separated. Furthermore, mud ponds containing the clay were water pollution hazards.
TVA endeavored to smelt raw phosphate ore that had been nodulized, but the operating characteristics of the phosphorus furnace were unsatisfactory. The furnace pressure fluctuations and gas temperatures were excessive and the slag was difficult to tap. Operation of the furnace was satisfactory when the percent P2O5 in the phosphate-plus-silica was 25-26, or greater. With unbeneficiated phosphate ore the percent P2O5 in the phosphate-plus-silica was in the range of 22-24. Florida hard rock containing 35.4 percent P2O5 was smelted at TVA without calcining or agglomeration to investigate its smelting characteristics. The P2O5 in the phosphate-plus-silica was 25.6 percent. The smelting characteristics of phosphate sand were compared with the smelting characteristics of phosphate sand that had been nodulized.
Results of tests comparing the smelting characteristics of the two feedstocks were reported in Chemical Engineering Report No. 3 as follows.                “The feeding of uncalcined Florida hard rock was tested at the modified No. 2 furnace, and the results of this test were compared with those of a preceding test in which nodulized Tennessee phosphate was used as a feed. Furnace operation was more erratic during the Florida hard rock test than it was during the nodulized phosphate test; the furnace pressure fluctuations were greater; the slag was more viscous and harder to tap; and precipitator operations were unsatisfactory. Deposits accumulated on the wires and frames of the precipitator after about 1 month's time and caused electrical short circuits which were not readily remedied by shaking or vibrating the frames. Inspection showed that some of the deposits were crystalline projections extending horizontally from the wires, and analysis showed that they consisted principally of P2O5 and SiO2. Although precipitator operating difficulties increased when hard rock was fed, the quantity of dust removed by the precipitator was only about half as much as was removed when nodules were fed. Precipitator operating difficulties similar to those encountered with hard rock were also encountered during tests with uncalcined Florida pebble; however, they were not as serious. These results indicate that these conditions (and the accompanying erratic furnace operation) are characteristics of the use of uncalcined phosphates as furnace feed.”        
Based on findings at TVA, feedstock for the electric furnaces should have at least 25 percent P2O5 in the phosphate-plus-silica for satisfactory operation of the furnace. If the furnace is equipped with electrostatic precipitators the weight ratio of fluorine to P2O5 should be a maximum of 0.089 to avoid accumulation of crystalline deposits on the wires and frames of the electrostatic precipitators. Florida hard rock was a sedimentary deposit with a ratio of F to P2O5 of about 0.110 and after TVA smelted the feedstock for one month accumulations of crystalline deposits on the wires and frames impaired dust collection and increased the proportions of elemental phosphorus that was recovered as phosphoric sludge. The composition of nodulized phosphate sand given in Table 1 show this feedstock contained 2.2 percent F and 27.5 percent P2O5 for a F/P2O5 of 0.080. The crystalline solids containing F and P2O5 did not accumulate on the wires and frames when nodulized phosphate sand was the feedstock.
TVA mined and beneficiated phosphate ore from mineral deposits in Maury and Williamson Counties in middle Tennessee until 1950. The raw phosphate ore was beneficiated by washing to remove clay and obtain phosphate sand which was nodulized to obtain feedstock having the composition shown in Table 1. Phosphate ore amenable to beneficiation by washing became exhausted. Losses of fluorapatite mineral that were washed out with clay were excessive. Furthermore, mud ponds were water pollution hazards.
The raw phosphate ore contained sufficient clay binder for agglomeration by heating in the temperature range of 2200° to 2350° F. Most of the clay in raw phosphate ore was washed out to prepare phosphate sand and it had to be heated to 2500° to 2700° F. for agglomeration. At TVA raw phosphate ore was agglomerated by heating in rotary kilns and it was agglomerated by heating in a traveling-grate kiln.
About 35 percent of the fluorine in phosphate sand was volatilized when it was agglomerated at 2600° to 2700° F. The F/P2O5 ratio in the feedstock was 0.080 as determined from Table 1 and 91.4 percent of the F was recovered in the slag when the feedstock was smelted. Accumulations of crystalline deposits on the wires and frames in the electrostatic precipitators was not a problem.
About 8 percent of the fluorine in raw phosphate ore was volatilized when it was agglomerated by heating to 2200 to 2350 F. The F/P2O5 ratio in the feedstock was 0.095 to 0.100 and a shut-down was required at frequencies of about three months for manual removal of the crystalline accumulations on the wires and frames. Electrostatic precipitators are not considered worthwhile if the phosphate ore is agglomerated by heating in the range of 2200° to 2350° F.
TVA smelted raw phosphate ore that was mined in Maury and Williamson Counties in Tennessee but the feedstock had to be upgraded by purchased flotation concentrate mined in Florida. The raw phosphate ore was beneficiated by washing, screening, and flotation to obtain flotation concentrate containing 30 to 32 percent P2O5. Flotation concentrate was incorporated in raw phosphate ore from deposits in middle Tennessee and the mixture was agglomerated by heating in rotary kilns and it was agglomerated by heating in a traveling-grate calciner. The cost of flotation concentrate was significantly higher than the cost of raw phosphate ore mined in middle Tennessee and freight costs from south Florida to north Alabama was more than the cost of the flotation concentrate. TVA demonstrated that raw phosphate ore could be smelted by upgrading feedstock to about 25 percent P2O5 in the phosphate-plus-silica but it was evident a lower cost process for upgrading the feedstock was needed.
When TVA stopped producing elemental phosphorus in 1976 the agency had mineral rights on phosphate land in Giles County, Tennessee. The mineral rights were sold to a company producing elemental phosphorus but I understand the land owners now possess mineral rights. The percent P2O5 and tons of raw phosphate ore on the various tracts in Giles County are in Table 2.
TABLE 2Giles County Phosphate PropertyP2O5 inTons of RawName of TractPercentPhosphate OreMcMillon, Harvey W.23.6478,000Consolidated Phosphate Co. (Brown)23.0200,350Consolidated Phosphate Co. (Kincaid-Hill)21.9673,950Foster, E. V. (Davis)23.1160,300Dunlap, T. F.22.0N.A.Dunlap, T. F.22.0120,000Gordon, L. R.22.5N.A.Gordon, L. R.22.5338,000Poorch, W. E.23.4389,100Opton, R. W.22.6893,100Clark, J. T. and G. N.22.2198,000Childers, Mrs. Ben24.9558,500Dunlap, W. P.22.2286,200Smith, J. P.23.682,900Upton, L. Q. (Whitworth and King)21.6532,000Yokley, John T.21.1450,500Whitworth, W. T.21.899,000Burgess, M. G.21.8862,250Burgess, L. B.19.5494,500Robinson, Robert20.6191,600Harwell, Riggs20.1102,800Phillips, Henry20.5288,000Upton and Keller20.51,197,152Topp, Gordon18.6257,300Burton, William R.19.8126,360Lanier, C. W.19.6133,600Smith. D. A.21.3193,100N.A.: Not Available 
As shown in Table 2 the P2O5 content of the various tracts varied from 18.6 to 24.9 percent and the weighted average was 20.82 percent P2O5. Phosphatic feedstocks, or mixtures of phosphatic feedstocks, should contain a minimum of about 25 percent P2O5 in the phosphate-plus-silica. The Giles County deposits are virtually self-fluxing; that is, the phosphate ore contains enough silica to combine with the CaO and Al2O3 to form a fluid slag and the percent P2O5 reported in Table 2 is the approximate percent P2O5 in the phosphate-plus-silica.
At TVA feedstock prepared from phosphate sand had a percent P2O5 in the phosphate-plus-silica of 25.8 and feedstock prepared from Florida pebble had a percent P2O5 in the phosphate-plus-silica of 25.4.
The primary object of my invention is to devise a practical, low-cost process for upgrading feedstock prepared from raw phosphate ore obtained by mining phosphate ore in Giles County, Tennessee, and Limestone County, Alabama. However, the invention may be applicable for upgrading feedstock prepared from other deposits of phosphate ore.